This invention relates to electrochemical cells and batteries. More particularly, it relates to lithium anode, thionyl chloride active cathode depolarizer cells and the use of additive films to enhance their performance.
The recent growth in portable electronic products requiring electrochemical power cells for energy has highlighted the definiencies of existing power cells for demanding applications. In an effort to improve the electrochemical cell technology, much attention has been given to perfecting what is generally referred to as lithium batteries. More precisely, this means electrochemical cells using a highly reactive anode such as lithium in combination with varying cathode and electrolyte materials. Indeed the literature is replete with examples of lithium anode cells having different cathodes and electrolytes. The interest in this cell technology stems from a recognition that such cells theoretically provide higher energy densities, higher voltages, wider temperature operating ranges, longer shelf lives, and lower cost.
Among all the known combinations of lithium anodes with different cathodes and electrolytes, those believed to have among the highest energy density and current delivery capability use certain inorganic liquids as the active cathode depolarizer. This type of cell chemistry is commonly referred to as liquid cathode.
The use of a liquid as an active cathode depolarizer is a radical departure from conventional cell technology. Until recently, it was generally believed that the active cathode depolarizer could never directly contact the anode. However, it has recently been discovered that certain active cathode materials do not react chemically to any appreciable extent with an active anode metal at the interface between the metal and the cathode material, thereby allowing the cathode material to contact the anode directly.
Early liquid cathode cells use liquid sulfur dioxide active cathode depolarizer and are described in U.S. Pat. No. 3,567,515 issued to Maricle, et al. on Mar. 2, 1971. Since sulfur dioxide is not a liquid at room temperature and at atmospheric pressure, it proved to be a difficult chemistry with which to work.
A major step forward in the development of liquid cathode cells was the discovery of a class of inorganic materials, generally called oxyhalides, that are liquids at room temperature. These materials perform the function of active cathode depolarizer. Additionally, they may also be used as the electrolyte solvent. Liquid cathode cells using oxyhalides are discribed in U.S. Pat. No. 3,926,669 issued to Auborn on Dec. 16, 1975 and in British Pat. No. 1,409,307 issued to Blomgren, et al. on Oct. 18, 1975. At least one of the oxyhalides, thionyl chloride (SOCl.sub.2), in addition to having the general characteristics discribed above, also provides substantial additional energy density and current delivery capability.
The liquid cathode systems, however, have suffered from two major problems that have prevented their wide spread use. First, they can be dangerous under certain circumstances, and secondly, high current rate batteries could not be made with good long term storage characteristics.
The first of these problems is addressed by copending application, Ser. No. 828,493, filed on Aug. 29, 1977 in the name of Louis R. Giattino and assigned to the same assignee as the present application.
The second problem is the subject of the present application. This problem manifests itself in two principal ways. First, there is a voltage delay after storage at elevated temperatures. That is, after cells have been stored at temperatures exceeding room temperatures, the cell voltage under moderate discharge loads requires some period of time, running between several seconds and several hours, to approach the level it would have instantly achieved prior to storage. Secondly, cells that can reliably deliver current rates of 7 or better ma per sq cm are difficult to achieve. Taken together, these two problems are generally referred to as the passivation phenomena.
Studies have indicated that passivation results from a build-up of compounds on the surface of the lithium anode. These compounds are not presentally well understood; however, it is generally believed that they are the product of a reaction between the lithium and one or more of the following: The electrolyte solvent, the electrolyte solute, or impurities particularly iron.
If cells are assembled carefully, passavation can be minimized in fresh batteries. However, storage, particularly at elevated temperatures, causes passivation to occure rapidly.
Others have attempted to solve the passivation problem in a variety of ways. One approach was to partially discharge cells prior to storage. A second approach was to increase the roughness factor of the anode by etching prior to assembly. A third approach was to pretreat the cathode current collector to eleminate impurities. Other approaches including the introduction of certain inorganic material such as water and sulfur dioxide have also been tried. However, to date, all known approaches have made little impact on the passivation problem.
It is therefore an object of this invention to reduce the passivation phenomenon in liquid cathode cells.
It is another object of the invention to provide an improvement in voltage delays for lithium anode, liquid cathode cells.
It is another object of the invention to provide improved current rate delivery capability for lithium anode, liquid cathode cells.
It is yet another object of this invention to reduce passivation in lithium anode, thionyl chloride cells.
Finally, it is an object of this invention to provide improved storage characteristics for lithium anode, liquid cathode cells.